I. Field of the Invention
The present invention relates generally to methods and devices for the noncontact monitoring and measurement of the torsional dynamics of rotating shafts. The present invention relates more specifically to methods and devices for the noncontact monitoring and measurement of stationary and transient torques in rotating shafts using magnetostrictive sensors (MsS).
II. Description of the Prior Art
It is common in many different machine systems, for mechanical power from an electrical motor, combustion engine, or gas turbine to be transmitted to a load through a power train of some type. Rotating shafts are frequently integral parts of such power trains. A variety of machinery dynamics, including vibration, lateral movement, and torsional motion, have a direct effect on the operational conditions of these machine systems. The monitoring of these machinery dynamics offers, therefore, a valuable means of diagnosing and correcting machinery problems in a manner that can assist in the effective operation and maintenance of the machinery.
For example, the measurement of the torsional dynamics of a rotating shaft can be used to control backlash at gear teeth and other types of drive train couplings, for a more efficient operation of the machinery and less wear on the machinery components.
At present, various methods are used to measure the torsional dynamics of a rotating shaft. These methods include shaft encoders, torsional accelerometers, and strain gauges. In general, the currently available methods require physical contact of some type with the rotating shaft (side or end surfaces) and a means of electrical/electronic communication such as slip rings or telemetry for relaying sensor information to signal analysis equipment. In many situations, these methods are not only difficult to use but costly to implement. In addition, the available methods generally lack long-term durability, which is essential for on-line monitoring and control during the service life of high speed rotational machinery.
Some efforts in the past have attempted to implement a non-contact means for the measurement of dynamic torsion in rotating shafts using magnetostrictive techniques. None of these efforts, however, disclose or anticipate a detector that does not include some form of periodic external excitation of the magnetostrictive material. The following patents are considered illustrative of the art encountered within the field.
U.S. Pat. No. 4,979,399, issued to Klauber et al. on Dec. 25, 1990, entitled xe2x80x9cSignal Dividing Magnetostrictive Torque Sensorxe2x80x9d, describes a non-contacting method for sensing torque utilizing the magnetostrictive principle by inducing a primary magnetic flux in a rotating shaft with an excitation coil.
U.S. Pat. No. 4,939,937, issued to Klauber et al. on Jul. 10, 1990, entitled xe2x80x9cMagnetostrictive Torque Sensorxe2x80x9d, likewise describes a system for sensing torque based on the magnetostrictive principle that utilizes a primary excitation coil to introduce a magnetic flux in the rotating shaft. The system involves appropriate placement both of a sensor coil and the primary excitation coil in positions adjacent to the rotating shaft and appropriate orientation of the coils with respect to each other.
U.S. Pat. No. 4,811,609, issued to Nishibe et al. on Mar. 14, 1989, entitled xe2x80x9cTorque Detecting Apparatusxe2x80x9d, describes a system for measuring the transmitted torque within a rotating magnetic material by means of a magnetostrictive sensor. Essential to the Nishibe system is the use of a demagnetization coil designed to restore the rotary magnetic material to a state of zero magnetization. Included in this system are driving circuits and excitation coils for establishing the magnetic field within the rotating shaft.
U.S. Pat. No. 4,803,885, issued to Nonomura et al. on Feb. 14, 1989, entitled xe2x80x9cTorque Measuring Apparatusxe2x80x9d, also describes a non-contact method for measuring torque in a rotating shaft of ferromagnetic material using magnetic based sensors. The device includes an excitation coil wound around the outer periphery of the rotating shaft and adapted to magnetize the shaft in an axial direction. A detecting core ring in the form of an integral unit includes a number of detecting cores arranged around the circumferential area of the rotating shaft.
U.S. Pat. No. 3,046,781, issued to Pratt on Jul. 31, 1962, entitled xe2x80x9cMagnetostrictive Torque Meterxe2x80x9d, provides an early teaching of the basic approach of employing magnetostrictive principles to implement a torque meter for a rotating shaft based on stress measurements of the shaft material. The description of the operation of the non-contacting embodiment (shown in FIG. 1 of the Pratt patent) refers to the use of an AC excitation coil wound around the shaft, and the inclusion of a xe2x80x9cmagnetostriction constantxe2x80x9d.
Japanese Patent No. 3-269228, issued to Aisin Seiki on Nov. 29, 1991, entitled xe2x80x9cMagnetostriction Detector for Torque Detector Using Film of Magnetostrictive Metal Containing Super Magnetostrictive Alloy Particlesxe2x80x9d, describes a system for measuring torque in a rotating shaft utilizing a primary excitation coil and a secondary detection coil adjacent to a surface on the shaft that has been covered with a ferromagnetic material. The focus of this patent involves the type of metallic material utilized as the magnetostrictive substance.
Each of the above patents describe devices for measuring torque in a shaft using a similar approach that requires a means for applying an AC magnetic field to the ferromagnetic shaft material. Most of the later issued U.S. patents provide teachings of similar magnetostrictive torque measuring approaches. Some of these patents suggest using a thin coating of magnetostrictive materials around nonferromagnetic materials as is well known in the field of magnetostrictive sensing.
III. Background on the Magnetostrictive Effect
The magnetostrictive effect is a property peculiar to ferromagnetic materials. The magnetostrictive effect refers to the phenomena of physical, dimensional change associated with variations in magnetization. The effect is widely used to make vibrating elements for such things as sonar transducers, hydrophones, and magnetostrictive delay lines for electric signals. The magnetostrictive effect actually describes physical/magnetic interactions that can occur in two directions. The Villari effect occurs when stress waves or mechanical waves within a ferromagnetic material cause abrupt, local dimensional changes in the material which, when they occur within an established magnetic field, can generate a magnetic flux change detectible by a receiving coil in the vicinity. The Joule effect, being the reverse of the Villari effect, occurs when a changing magnetic flux induces a mechanical vibrational motion in a ferromagnetic material through the generation of a mechanical wave or stress wave. Typically, the Joule effect is achieved by passing a current of varying magnitude through a coil placed within a static magnetic field thereby modifying the magnetic field and imparting mechanical waves into a ferromagnetic material present in that field. These mechanical or stress waves then propagate not only through the portion of the ferromagnetic material adjacent to the generating coil but also into and through any further materials in mechanical contact with the ferromagnetic material. In this way, non-ferromagnetic materials can serve as conduits for the mechanical waves or stress waves that can thereafter be measured by directing them through these ferromagnetic xe2x80x9cwave guidesxe2x80x9d placed proximate to the magnetostrictive sensor element.
The advantages of magnetostrictive sensors over other types of vibrational sensors becomes quite clear when the structure of such sensors is described. All of the components typically utilized in magnetostrictive sensors are temperature, pressure, and environment-resistant in ways that many other types of sensors, such as piezoelectric based sensors, are not. High temperature, permanent magnets, magnetic coils, and ferromagnetic materials are quite easy to produce in a variety of configurations. Further, although evidence from the previous applications of magnetostrictive sensors would indicate the contrary, magnetostrictive sensors are capable of detecting mechanical waves and translating them into signals that are subject to very fine analysis and discrimination in a manner that allows information to be obtained about the elements in an object (such as a rotating steel shaft) that may have initially generated the stress.
It would be desirable, therefore, to have a torque measurement system that utilizes magnetostrictive sensors in conjunction with a rotating shaft. It would be desirable to maintain the advantages of such a system through its non-contact method of detecting the magnetostrictive effect within ferromagnetic material contained on or in the rotating shaft. In addition, it would be preferable to simplify such a system by eliminating the need for at least the primary excitation coil found in each of the existing systems based on magnetostrictive sensors. It would be desirable to implement such a system with a magnetostrictive sensor that provides a signal which, when amplified and appropriately filtered, carries the same information about the torque being experienced in the rotating shaft as more expensive, cumbersome, and delicate systems that use strain transducers, telemetry devices and the like.
The present invention involves methods and devices for the non-contact measurement of dynamic torsion in a rotating shaft using magnetostrictive sensors (MsS). The present invention utilizes a specially configured signal detector, that includes an inductive pickup coil, in which signals corresponding to localized shaft torques are induced. The techniques of the present invention are particularly advantageous for the active monitoring of loaded rotating shafts that are integral parts of power trains, by providing a low-cost and long-term sensor for acquiring dynamic data of the shaft portion of the machinery system being monitored and/or controlled.
As disclosed in the basic illustrative embodiment, an inductive pickup coil is positioned to encircle the rotating shaft whose torsion dynamics are to be measured. Dynamic stresses associated with torsional vibrations of the shaft cause changes in the magnetic induction of the magnetostrictive material of which the shaft is made (or plated with), which in turn induce signal voltages in the pickup coil. A permanent biasing magnet positioned outboard of the pickup coil maintains the magnetostrictive material magnetized, by biasing it, or in the alternative, leaving a residual magnetization in the shaft area adjacent the signal detector. Keeping the magnetostrictive material magnetized increases the stress sensitivity of the detector and makes its frequency response linear with stress. In use, the detected signals are conditioned using standard electronic signal conditioning circuitry for subsequent processing in a data processor to develop the desired dynamic torsional data.